The antigen/antibody reaction has long been known in the art. The highly specific nature of this reaction has led to its widespread application in both qualitative and quantitative asays. See, Skelly, D. S. et al. Radioimmunoassays, Clin. Chem., Vol. 19, pp. 146-86 (1973).
Stated in the most general terms, assay techniques which use the antigen/antibody reaction call for the "raising" of antibodies to an antigen, such as by injecting the antigen into the bloodstream of a rabbit or other suitable animal. The animal's immune system responds to the presence of the antigen by producing antibodies to the antigen. These antibodies are then harvested from the animal. The antibodies can be used in an assay procedure for the presence of the antigen (or the presence of its immunological equivalent) by mixing them with a sample which is thought to contain that compound.
This technique is not without it limitations. Among these is the difficulty which is encountered in raising antibodies to small molecules. Most small molecules fail to provoke an immune reaction (i.e. the production of antibodies) when they are introduced into the bloodstream of an animal. This is the case even though these compounds are often very toxic. It is believed that the mammalian immune system cannot "see" them and thus does not respond to them. Also, these molecules may be so very small that they cannot provide a sufficiently large "binding site" for antigen/antibody binding.
It is sometimes possible to surmount the first of these difficulties by attaching the molecule of interest to a larger molecule. In this case, the molecule of interest, which is called a hapten, is attached to a molecule such as bovine serum albumin (BSA) protein and then injected into the bloodstream of an animal. BSA is large enough to be "seen" by the immune system and the immune system reacts to the BSA and the hapten. This technique often result in the production of antibodies to the hapten. However, its effectiveness is not universal, and it has not been found to be effective in the production of antibodies to very small molecules or to specific small functional groups. This is thought to be because they are so small that antigen/antibody binding is difficult or impossible.